In the second century, Galen recognized that nerve and muscle were
functionally inseparable since contraction of muscle occurred only if
the nerves supplying that muscle were intact. He therefore concluded
that the shortening of a muscle was controlled by the central nervous
sytem while the extension of a muscle could occur in the absence of
innervation. Nerves, he thought, were the means of transport for animal
spirits to the muscles; the way in which animal spirits may bring about
contraction dominated the study of muscle physiology from that time
until the historical discovery of Galvani that muscle could be
stimulated electrically and that nerve and muscle were themselves a
source of electrical energy. It is now well known that nerves conduct
electrically and that transmission from nerve to striated muscle is
mediated by the chemical which is liberated from nerve terminals onto
the muscle membrane. In vertebrates this chemical is acetylcholine
(ACh). Thus the concept of spirits that are released from nerves and
control muscle contraction directly, is no longer tenable. Nevertheless
the concept of 'substances' transported down nerv s which directly
control many aspects of muscle has not been abandoned, and has in fact
been frequently reinvoked to account for the long-term regula- tion of
many characteristics of muscle (see review by Gutmann, 1976) and for the
maintenance of its structural integrity.